Practice Test 12 - MCAT Chemical and Physical Foundations of Biological Systems

20 questions. Answer them in any order, then submit to see your score and every explanation.

A neuron's membrane can be approximated as a capacitor with capacitance $C$ in parallel with ion channels. During a brief current clamp, a constant current $I$ is injected for a very short time such that channel conductances do not significantly change. The initial rate of membrane potential change satisfies $\frac{dV}{dt} = \frac{I}{C}$ . Which statement best describes the relationship between capacitance and voltage dynamics in this biological context?
A student tests coherence of two laser diodes for an optical trapping setup by sending each through the same double-slit apparatus (slit separation $d$ fixed, screen distance $L$ fixed). Laser 1 has wavelength $\lambda_1 = 450\ \text{nm}$ and Laser 2 has wavelength $\lambda_2 = 650\ \text{nm}$ . Both beams have similar power and illuminate the slits uniformly. For small angles, fringe spacing on the screen scales as $\Delta y \propto \lambda L/d$ . Which observation is most consistent with the interference pattern difference between the two lasers?
During a biceps curl, a forearm is held stationary with the elbow flexed. Model the forearm-hand segment as a rigid body in static equilibrium. External forces on the segment include its weight $W$ acting downward at the center of mass, an upward muscle force $F_m$ from the biceps tendon applied near the elbow, and a joint reaction force at the elbow with horizontal and vertical components. Ignoring any acceleration, based on Newton's Laws which statement is most consistent?
A dimeric enzyme requires proper quaternary structure to form a complete active site at the subunit interface. A mutation disrupts dimerization but leaves each monomer folded. The mutant shows a large decrease in $V_{max}$ and no measurable change in $K_m$ for the small fraction that remains dimeric. Which structural change would most likely increase enzyme activity?
Submitochondrial particles (inside-out inner mitochondrial membrane vesicles) were prepared so that the F1 domain of ATP synthase is exposed to the external solution. The vesicles were supplied with NADH (200 µM) and oxygen, and proton translocation was inferred from external pH change. A separate condition included oligomycin, which blocks the F0 proton channel of ATP synthase. Based on the vignette, which outcome is most consistent with energy conservation principles?
A small amphibian egg is placed in a vertical cylinder containing two immiscible layers: an upper oil layer (density $0.80\ \text{g/mL}$ ) and a lower aqueous layer (density $1.05\ \text{g/mL}$ ). The egg's density is $0.95\ \text{g/mL}$ and it does not adhere to the walls. After release, it reaches static equilibrium. Based on buoyancy in stratified fluids, which outcome is most likely?
A lab cart carries a vertical pole with a horizontal arm (length 0.50 m) that can pivot at the pole. A 4.0-N mass hangs from the arm's end, creating torque about the pivot. To reduce the tendency of the arm to rotate downward without changing the hanging mass, which change is most consistent with torque principles?
A puck slides along a straight air table. At $t=0$ , its velocity is $+3.0\ \text{m/s}$ . After 2.0 s, its velocity is measured as $-1.0\ \text{m/s}$ . Assuming constant acceleration over this interval, which statement best describes the acceleration direction?
An electrophysiology rig models a cell membrane as a capacitor. A membrane patch is approximated as $C=20\ \text{pF}$ and is initially at $V_m=-70\ \text{mV}$ . A brief current pulse transfers $+2.0\ \text{pC}$ of charge onto the membrane capacitor (sign defined as increasing the inside potential). Based on $\Delta V=\Delta Q/C$ , which outcome is most consistent with the change in electric potential? (Constants: $1\ \text{pC}=10^{-12}\ \text{C}$ .)
A researcher studies osmotic flow across a semipermeable membrane that passes water but not sucrose. Side 1 contains 0.10 M sucrose; Side 2 contains 0.20 M sucrose. Temperature is held at 298 K, and sucrose is treated as a non-electrolyte ( $i=1$ ). The osmotic pressure difference is approximated by $\Delta\pi = iRT\Delta C$ , with $R=0.082$ L·atm·mol $^{-1}$ ·K $^{-1}$ . Which prediction is most consistent with the model for the direction of net water movement and the sign of $\Delta\pi$ (defined as $\pi_2-\pi_1$ )?
Two nonreacting gases are compared at the same temperature and pressure: CO2 and SO2. Both are small molecules, but SO2 has a bent geometry and a permanent dipole, while CO2 is linear and nonpolar. Which statement best reflects the impact of dipole–dipole interactions on condensation (liquefaction) tendency under these conditions?
A lab ran isoelectric focusing (IEF) on a serum fraction, then cut a lane and ran it in a second dimension SDS-PAGE (2D electrophoresis). In IEF, proteins focused at positions corresponding to pH values. Four spots were identified: Spot | IEF position (pH) | SDS-PAGE MW (kDa) 1 | 4.9 | 66 2 | 6.8 | 66 3 | 8.9 | 66 4 | 6.8 | 25 Which statement is most consistent with the electrophoresis data?
In a tracer study, a peptide is synthesized with sulfur as either $^{32}\text{S}$ or $^{34}\text{S}$ . Which statement is most consistent with the relationship between these isotopes?
A blood-gas analyzer is calibrated using a bicarbonate buffer where $pK_a = 6.1$ for the $\mathrm{H_2CO_3}/\mathrm{HCO_3^-}$ pair. Two calibration solutions are prepared at 37°C: Solution 1: $[\mathrm{HCO_3^-}] = 20\ \mathrm{mM}$ , $[\mathrm{CO_2(aq)}] = 1.0\ \mathrm{mM}$ Solution 2: $[\mathrm{HCO_3^-}] = 10\ \mathrm{mM}$ , $[\mathrm{CO_2(aq)}] = 1.0\ \mathrm{mM}$ Assume $[\mathrm{H_2CO_3}]$ tracks with $[\mathrm{CO_2(aq)}]$ and the Henderson–Hasselbalch relationship applies qualitatively. Which statement best describes the system's response when moving from Solution 1 to Solution 2?
During water softening, hard water containing Ca $^{2+}$ is treated with a sodium-form cation-exchange resin: 2(R–SO $_3^-\,\text{Na}^+)(s) + Ca$ ^{2+}$(aq) $\rightleftharpoons$ (R–SO $_3^-)_2\,\text{Ca}^{2+}$ (s) + 2 Na $^+$ (aq). Which outcome would be expected from the described ion interaction?
A research group reconstituted two synthetic vesicle membranes to model a eukaryotic plasma membrane inner leaflet. Vesicle X contained mostly phosphatidylcholine with two saturated $18{:}0$ acyl chains; Vesicle Y contained mostly phosphatidylcholine with two monounsaturated $18{:}1$ acyl chains. Both vesicle types were tested at $37\,^{\circ}\mathrm{C}$ for passive permeability to a small neutral solute (urea) under identical concentration gradients. Based on bilayer organization and packing, which outcome is most consistent with these compositions?
A researcher shines monochromatic light onto a clean metal surface in vacuum and measures emitted electrons. With 450 nm light, electrons are emitted; with 650 nm light at the same intensity, no electrons are emitted. Increasing the 650 nm intensity does not produce emission. Constants: $c = 3.0\times10^8\ \text{m/s}$ , $h = 6.63\times10^{-34}\ \text{J}\cdot\text{s}$ . Which conclusion about light's behavior is most consistent with these results?
A mass spectrometer produces a peak at $m/z = 23$ from a salt sample under conditions that yield primarily singly charged atomic ions. (Principle: for $z=+1$ , $m/z$ approximates atomic mass in u.) Based on the spectra, which atom is most likely present? Constants: none needed.
A microfluidic actuator does $W = 4.0\times10^{-3}\ \text{J}$ of work on a flexible membrane during one cycle. It runs at $f = 50\ \text{Hz}$ (50 cycles per second). The core principle tested is power from work per cycle: $P = W\times f$ . Which value is most consistent with the actuator's average power output?
A lab tracks the size of isoelectronic ions to interpret migration in an electric field. Consider the set: O2−, F−, Na+, Mg2+. All have 10 electrons. Based on periodic trends, which ion should have the smallest radius? Assume radii are dominated by nuclear charge for isoelectronic species.

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